Site-specific accumulation of a LINE-like retrotransposon in a sex chromosome of the dioecious plant Cannabis sativa

Male-associated DNA sequences were analysed in hemp (Cannabis sativa L.), a dioecious plant with heteromorphic sex chromosomes. A male-associated DNA sequence in C. sativa (MADC1) and its flanking sequence encoded a reverse t ranscriptase that was strongly homologous to those of LINE-like retrotransp osons from various plants and other organisms, as well as another open read ing frame (ORF). Fluorescence in situ hybridization (FISH) with MADC1 as pr obe, which yielded strong signals specific for male genomic DNA in gel blot analysis, generated a clear doublet signal at the end of the long arm of t he Y chromosome. FISH using pachytene chromosomes of pollen mother cells at meiotic prophase I revealed that pairing of X and Y chromosomes occurred a t the short arm of the Y chromosome where MADC1 was not present. Furthermor e, FISH using extended DNA fibers, with MADC1 and its flanking DNA as probe s, revealed that 100 to 200 copies of the retrotransposon were located in t andem on the Y chromosome. These results support the hypothesis that accumu lation of a specific LINE-like retrotransposon at the terminal region of th e long arm of the Y chromosome might be one cause of heteromorphism of sex chromosomes.